Data from: Sensitivity of the farmland bird community to crop diversification in Sweden: does the CAP fit?

Crop diversification has been introduced as an environmental strategy in the ‘Greening’ of the EU Common Agricultural Policy (CAP) for 2015–2020. The primary target of crop diversification is soil and ecosystem resilience, but claims for potential benefits for farmland biodiversity are also common. However, understanding of relationships between the number (compositional heterogeneity) and spatial arrangement (configurational heterogeneity) of crop fields and biodiversity is generally poor, making such claims relatively unfounded. In this study, we monitored crop and farmland bird diversity on 178 farms across Sweden's main agricultural areas. From a pre-implementation assessment, we show that >97% of the assessed farms would not be required to change their management under the CAP crop diversification measure (minimum of three crops for farms with 30+ ha), suggesting that this measure has generated little change on Swedish farms. While accounting for non-crop elements and farming system (conventional or organic), we show that crop structural diversity (i.e. the management and vegetation structure of crops) rather than crop diversity senso lato positively affected richness of non-crop breeding bird species with stronger effects in arable, compared with forest-dominated landscapes. No such effects were observed among field-nesting farmland bird species. Organic farming had little influence on farmland birds with positive effects only in the most arable-dominated landscapes and for field-nesting species only. In forest-dominated landscapes, organic farms even held lower field-nester densities compared with conventional farms, possibly due to the dominance of grasslands on organic farms that in these landscapes support lower densities of field-nesting species compared with cereals. Policy implications. Our study illustrates the importance of a consideration of structural instead of species diversity of crops for biodiversity, in this case farmland birds. We also underline the absence of such a distinction in current EU Common Agricultural Policy Greening, while simultaneously setting levels on crop diversification too low resulting in little to no change in landscape-scale crop diversity on Swedish farmland. We recommend that future efforts to manage farmland biodiversity should include ways of increasing the structural diversity of crops at the scale of farms and landscapes.

欧盟共同农业政策（Common Agricultural Policy, CAP）2015-2020年的绿色化改革中，引入了作物多样化作为环境策略。作物多样化的核心目标是提升土壤与生态系统韧性，但关于其对农田生物多样性具有潜在益处的主张也屡见不鲜。然而，目前学界对作物田的数量（组成异质性，compositional heterogeneity）与空间配置（配置异质性，configurational heterogeneity）和生物多样性之间的关联认知普遍不足，使得此类主张相对缺乏依据。

本研究对瑞典主要农业区域的178个农场开展了作物与农田鸟类多样性监测。通过实施前评估，我们发现，依据CAP作物多样化措施（30公顷以上农场需至少种植3种作物），超过97%的受访农场无需调整其种植管理模式，这表明该措施在瑞典农场中几乎未产生实际变革。

在纳入非作物生境要素与耕作体系（常规或有机农业）的变量后，我们发现，对非作物生境繁殖鸟类物种丰富度产生正向影响的是作物结构多样性（即作物的管理方式与植被结构），而非广义作物多样性；且该影响在耕地主导的景观中比森林主导的景观更为显著。而在地面筑巢的农田鸟类类群中，并未观察到此类关联。

有机农业对农田鸟类整体影响微弱，仅在耕地占比最高的景观中对地面筑巢鸟类存在正向影响。在森林主导的景观中，有机农场的地面筑巢鸟类种群密度甚至低于常规农场，这可能是因为有机农场以草地为主导生境，而在这类景观中，相较于谷类作物生境，草地所能支撑的地面筑巢鸟类种群密度更低。

政策启示。本研究阐明，在保护生物多样性（此处以农田鸟类为例）时，应优先考量作物结构多样性而非物种多样性。同时我们也指出，当前欧盟共同农业政策绿色化改革并未纳入此类区分标准，且其设定的作物多样化门槛过低，导致瑞典农田在景观尺度上的作物多样性几乎未发生任何变化。我们建议，未来在开展农田生物多样性管理工作时，应纳入提升农场与景观尺度下作物结构多样性的相关路径。